added books model

nishaespera_reading/bookshelf/models.py

 from django.db import models
+from datetime import datetime
+from django.core.validators import MaxValueValidator, MinValueValidator
+from django.core.exceptions import ValidationError
 
 class Author(models.Model):
     first_name = models.CharField(max_length = 50)
@@ -6,3 +9,21 @@ class Author(models.Model):
     age = models.IntegerField(default = 0)
     nationality = models.CharField(max_length = 50)
     bio = models.TextField(max_length = 700)
+
+def require_int(value):
+    if value.isdigit() == False:
+        raise ValidationError('ISBN must only be integers.')
+    
+class Books(models.Model):
+    title = models.CharField(max_length = 70)
+    author = models.ForeignKey(Author, on_delete = models.CASCADE)
+    publisher = models.CharField(max_length = 70)
+    year_published = models.IntegerField(null = True,
+        validators=[MinValueValidator(1400), MaxValueValidator(datetime.now().year)]
+        )
+    ISBN = models.CharField(max_length = 13, validators = [MinValueValidator(13), require_int])
+    blurb = models.CharField(min_length = 100, max_length = 200)
+
+
+
+